In the Unity editor on Mac OS X.

	if (Application.platform == RuntimePlatform.OSXEditor)
		Debug.Log("Do something special here!");

using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    void Example() {
        if (Application.platform == RuntimePlatform.OSXEditor)
            Debug.Log("Do something special here!");
        
    }
}

import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	def Example() as void:
		if Application.platform == RuntimePlatform.OSXEditor:
			Debug.Log('Do something special here!')